Heisenbug on database replicator tests.

## Environment details:

* local deployment, vagrant, native ubuntu, Mac OS, Bluemix, ...
* version of docker, vagrant, ubuntu, ...

## Steps to reproduce the issue:

Heisenbug on travis test, will try to reproduce this later...

## Provide the expected results and outputs:

Test `whisk.core.database.test.ReplicatorTests » Database replication script should replay a database` should be passed.

## Provide the actual results and outputs:

Logs from gradle scan.
```
Removing database: backup_1532879332_replicatortest_whisk_local_database_to_be_restored
Creating database: backup_1532879332_replicatortest_whisk_local_database_to_be_restored
Creating testdocument
Running replay: http://whisk_admin:some_passw0rd@172.17.0.1:5984, http://whisk_admin:some_passw0rd@172.17.0.1:5984, backup_1532879332_
expected exit code = 0
exit code = 1
stdout:
stderr: Traceback (most recent call last):
File "/home/travis/build/apache/incubator-openwhisk/ansible/../tools/db/replicateDbs.py", line 144, in
arguments.func(arguments)
File "/home/travis/build/apache/incubator-openwhisk/ansible/../tools/db/replicateDbs.py", line 113, in replayDatabases
sourceDb.create("_replicator")
File "/home/travis/.local/lib/python2.7/site-packages/couchdb/client.py", line 221, in create
self.resource.put_json(name)
File "/home/travis/.local/lib/python2.7/site-packages/couchdb/http.py", line 578, in put_json
**params)
File "/home/travis/.local/lib/python2.7/site-packages/couchdb/http.py", line 596, in _request_json
headers=headers, **params)
File "/home/travis/.local/lib/python2.7/site-packages/couchdb/http.py", line 592, in _request
credentials=self.credentials)
File "/home/travis/.local/lib/python2.7/site-packages/couchdb/http.py", line 427, in request
raise PreconditionFailed(error)
couchdb.http.PreconditionFailed: (u'file_exists', u'The database could not be created, the file already exists.')
```

## Additional information you deem important:
* issue happens only occasionally or under certain circumstances
* changes you did or observed in the environment